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Turkish National Fundamental GPS Network (TUTGA)
Turkish National Fundamental GPS Network (TUTGA) consisting of about 600 stations has been established through surveys between 1997 and 1999. General distribution of TUTGA stations is given in Figure 1. Following the establishment of TUTGA, revision surveys have been done due to the high seismicity after 1999 (Izmit Eq., 1999, Mw = 7.5, Düzce Eq., 1999, Mw = 7.2, Çerkes Eq., 2000, Mw = 6.1, Sultandag Eq., 2000, Mw = 5.9, Çay Eq., Mw= 6.6 and Bingöl Eq., 2003, Mw = 6.4) and due to the possibility of being destroyed. A revision plan has been put into practice which comprises reconnaissance and revision of all the TUTGA sites. For each station, 3D Coordinates and their associated velocities were computed in ITRF2000 and transformed into ITRF-96 which is the initial reference frame chosen for TUTGA. Definition of a national reference system called TURES-96 (Turkish National Reference System-1996) is still in progress in coordination with the Turkish National Permanent GPS Network. Positional accuracies of the stations are about 1-3 cm whereas the relative accuracies are within the range of 0.1 - 0.01 ppm. Besides, the network has been connected to the Turkish Horizontal and Vertical Control Networks through overlapping stations and time-dependent coordinates of all stations are being computed in the context of the maintenance of the network with repeated GPS observations. Also appropriate models for coordinate transformation from ED-50 system into the WGS84 have been defined in the context of TUTGA. Combining permanent and survey-type GPS measurements is still an on-going effort. Significant progress has been made and preliminary results were obtained.
Distribution of TNFGN stations.
Episodic GPS Observations for Geodynamics Studies
Interseismic deformation is monitored by periodic GPS and leveling measurements across Turkey while specific densified networks are established for local and regional secular deformation in certain regions. Velocity solution of GPS data over the interval 1992- gives the information for tectonic nature of Anatolia and its surrounding regions (Figure below).
Horizontal velocity field of Turkey and surrounding regions in a Eurasia-fixed frame.
Analyses of velocity field have been carried out to expose areas of secular deformation and seismic hazard. Strain analyses utilizing secular movements shed light on rigid block rotations, local compression and faulting areas that well conform to the geological and geophysical evidence of Anatolia. Due to high seismic activity, co-seismic and post-seismic deformation is also monitored by independent GPS campaigns. Earthquakes with magnitude equal and higher than Mw 6.0 cause surface displacements that should be taken into account in high-precision geodetic studies. Six such earthquakes have occurred since the establishment of Turkish National Fundamental GPS Network. Co-seismic surface displacements obtained from survey-type pre-earthquake and post-earthquake GPS observations are analyzed and modeled in an elastic isotropic medium. Depending on the time interval, computed inter-seismic deformation is dispersed from observed co-seismic deformations and published to civilian users surveying in the regions under earthquake influence. TUTGA as well as other existing stations comprises a set of precise coordinates along with their velocities and possible co-seismic corrections for the earthquake prone areas. Specifically, certain parts of Anatolia are still investigated through permanent and survey-mode GPS measurements in collaboration with international earth scientists.
In 2017, Tech-2000 surveying kits were procured from UNAVCO and classical tripods are not used anymore. Due to vandalism and other effects, pillars are destroyed more in number than bench marks on the bedrocks. To maintain a site for longer years, bench marking is a good solution. But it has disadvantages like centering and antenna height measurement errors. To overcome these difficulties, GDM surveyors stopped using tripods and started using tech-2000 surveying kits (Fig.).
Tech-2000 surveying kit.